In the case, in particular, of very-high-resolution Earth observation missions by satellite either in a low orbit (called “LEO orbit”), at an altitude lower than a few thousand kilometers, or in a high orbit (up to geostationary orbit, called “GEO orbit”), the line of sight of the observation instruments used must have very high stability.
Observation satellites generally have an attitude control system that comprises one or more inertial actuators to align the line of sight of an observation instrument of the satellite with a desired direction, and to stabilize the attitude of this observation instrument during image capture.
For reaction wheel types of inertial actuators, it is known that the rotation of the reaction wheels' unbalance create vibrations that propagate up to the observation instrument's line of sight and degrade the quality of the images. The higher the torque and angular momentum capacity of the reaction wheels used are—and therefore the higher the mass and speed of the rotor—the stronger the vibrations are.
This leads to contradictory requirements in the case of satellites that must execute fast and frequent tilting maneuvers to increase the number of image captures. Indeed, to execute fast maneuvers, high capacity reaction wheels have to be used, which produce strong vibrations during image capture.
In order to limit the vibrations caused by reaction wheels, it is known, in particular from the international application WO 2007/077350, to mount these reaction wheels on a damper that attenuates the amplitude of the vibrations transmitted to the observation instrument. However the vibrations remaining after attenuation are still too strong for the requirements of very-high-resolution observation missions.
It is also known, from the U.S. Pat. No. 6,758,444, to use a number of reaction wheels strictly greater than the number of axes along which the attitude of the satellite is to be controlled, providing at least one additional degree of freedom for controlling the satellite. This additional degree of freedom is used to minimize a cost function representative of the level of vibrations caused by the reaction wheels. However, this method requires knowing an accurate model of the vibrations caused by the reaction wheels. In practice, it is difficult to obtain such a model with sufficient precision, such that the performance will be generally limited and insufficient for the requirements of very-high-resolution observation missions.